Semiconductors are generally considered far superior to metals as thermoelectric materials because of their much larger Seebeck coefficients (). However, a maximum value of in a semiconductor is normally accompanied by a minuscule electrical conductivity (σ), and hence, the thermoelectric power factor ( = σ) remains small. An attempt to increase σ by increasing the Fermi energy (), on the other hand, decreases . This trade-off between and σ is a well-known dilemma in developing high-performance thermoelectric devices based on semiconductors. Here, we show that the use of metallic carbon nanotubes (CNTs) with tunable solves this long-standing problem, demonstrating a higher thermoelectric performance than semiconducting CNTs. We studied the dependence of , σ, and in a series of CNT films with systematically varied metallic CNT contents. In purely metallic CNT films, both and σ monotonically increased with , continuously boosting while increasing . Particularly, in an aligned metallic CNT film, the maximum of was ∼5 times larger than that in the highest-purity (>99%) single-chirality semiconducting CNT film. We attribute these superior thermoelectric properties of metallic CNTs to the simultaneously enhanced and σ of one-dimensional conduction electrons near the first van Hove singularity.